In U.S. Pat. No. 4,309,603 of Norman L. Stauffer, filed Oct. 17, 1979, a low cost auto focus system is disclosed in which as few as two radiation responsive detector pairs are utilized thus avoiding the necessity for a large and costly detector array that had been common in the prior art at that time. The purpose for this system was to provide an auto focus arrangement for less expensive cameras while still retaining most of the advantages obtained in the prior art. The system utilized the equation V=.vertline. log a.sub.1 /b.sub.2 .vertline.-.vertline. log a.sub.2 /b.sub.1 .vertline. where V is the resultant output and a.sub.1, b.sub.1, a.sub.2 and b.sub.2 are the respective outputs of the two pairs of detectors used. This equation operated to produce a summation curve that was normally positive on one side of the proper focus position and negative on the other side of the proper focus position with the zero crossover point representing the position of best focus.
In U.S. Pat. No. 4,250,377 of Dennis J. Wilwerding, filed Nov. 30, 1979, an improvement on the basic low cost auto focus system was disclosed in which a bias signal was introduced to change the position of the output signal by an amount necessary to prevent improper zero crossover situations that could occur under very low contrast conditions.
One difficulty has been encountered in the operation of the above-described inventions and that lies in the fact that the signals log a.sub.1 /b.sub.2 and log a.sub.2 /b.sub.1 may individually be quite large even near the zero crossover point representing the best focus position. Because of this, the difference amplifiers utilized with respect to the two detector pairs had to be extremely closely matched so that the difference between the absolute values of the log functions, represented by the above equation, can be zero at the proper position. It has been found that providing such closely matched amplifiers is a serious problem in the production of a low cost circuit and accordingly, schemes have been devised in an attempt to utilize amplifiers that are not necessarily exactly matched. One such scheme has been to utilize the equation .vertline. log a.sub.1 /b.sub.1 .vertline.+.vertline. log a.sub.2 /b.sub.2 .vertline. and instead of detecting the zero crossover point, determining the point at which the curve generated by this equation peaks. Such a circuit would avoid the necessity for closely matched amplifiers but introduces the problem of the difficulty of peak detection. Most hand-held cameras cannot be absolutely rigidly positioned during the determination of range and slight motions introduced by the operator cause small peaks to exist along the output curve. When this occurs, most peak detection circuits are unable to determine the difference between a small spurious peak and the actual peak representing best focus.
An alternate solution is disclosed in a copending application of J. Frazier, Ser. No. 212,930, filed Dec. 4, 1980. This system examined the magnitude of the signal log a.sub.1 /b.sub.1 to determine if it was greater or lesser than the signal log a.sub.2 /b.sub.2 and then, using only the larger signal, to plot a zero crossover point from the expression V=log a.sub.1 /b.sub.1 or V=log a.sub.2 /b.sub.2. Such an equation operates satisfactorily with large signals but the signal was found to be too small in some cases and furthermore, the information contained in the non-used signal was lost.